High oxygen utilization in BOD-containing water treatment

ABSTRACT

BOD-containing water such as municipal waste is mixed with at least 60% oxygen gas and active biomass in accordance with specified relationships of oxygen feed gas quantity/energy supplied ratio, oxygen concentration range in the aeration gas, and degree of oxygen saturation in the mixing liquor.

CROSS-REFERENCES TO RELATED APPLICATIONS

The following applications relating to oxygenation of BOD-containingwater were filed simultaneously with this application:

Ser. No. 838,442, cyclic Oxygenation of BOD-Containing Water, J. R.McWhirter; Ser. No. 838,498, Bio-chemical Oxidation With Low sludgeRecycle, E. K. Robinson and J. R. McWhirter; Ser. No. 838,499,Bio=Oxidation With Low Sludge Yield, J. R. McWhirter; Ser. No. 838,500,Staged Oxygenation of BOD-Containing Water, J. R. McWhirter.

BACKGROUND OF THE INVENTION

This invention relates to a method for treating BOD-containing water byoxygenation. The BOD-containing water may for example be municipalwaste, chemical waste from petrochemical or paper plants, orfermentation liquor.

Biochemical oxidation methods employ aerobic bacteria to convert varioussubstrates and nutrients to other forms of matter. A common example isthe activated sludge method for purifying sewage and industrial wastes.In such methods, the type and rate of reactions which occur arecritically dependent upon the presence of ample oxygen for use by thebacteria. The oxygen is made available to the bacteria by dissolutioninto the liquor from an aerating gas, and by uptake of the dissolvedoxygen (DO) by the bacteria.

High DO levels in the liquor are desirable for several reasons. Forexample, anaerobic zones are avoided and the rate of the biochemicalreaction is not hindered from a lack of oxygen. Moreover, the bacterialpopulation is improved by high DO levels because the growth of anaerobicand facultative strains is suppressed. Such strains cause odors andextend treatment time. Under certain conditions including high DO, abacterial floc is formed which settles more rapidly to higher densities.This produces an improved effluent and renders the BOD-containing watertreatment system less susceptible to upsets. Another desirablecharacteristic is that the large, desirable floc particles are moreadequately supplied with oxygen throughout their mass because the DOgradient supplied through the particle is higher. Finally, high DO inthe liquor in the system means that higher solids levels can besustained with resultant higher treatment rate and lower production ofexcess sludge.

Air is the common source-gas for dissolution of oxygen into the liquor.A common dissolution technique is to sparge or diffuse compressed airinto lower levels of open treatment tanks filled with a mixture ofliquid and bacterial solids (mixed liquor). The sparge air serves thedual purposes of creating a large gas-liquor interfacial area fordissolution of oxygen, and of stirring the mixed liquor so that thesolids remain in uniform suspension. For municipal sewage treatment,about 500 to 700 cu. ft. of air is usually diffused per lb. BOD removedfrom the influent water, and with 4 to 8 hours solids retention time,this corresponds to about 110-150 cu. ft. air per hour per 1000-gallonaeration tank capacity. Of the oxygen contained in this air, about 10%is dissolved and utilized in the biochemical oxidation and theremaninder is wasted. The amount of air which needs to be introducedsolely to keep the solids in suspension is on the order of 70-80 c.f.h.per 1000-gallon tank capacity, and is substantially less than thatactually introduced in order to dissolve sufficient oxygen. Hence, it isseen that the amount of air supplied is dictated by "oxygen demand," andthe amount of air is very large because of the low fraction of itscontained oxygen which can be dissolved. The air is compressed to alevel determined by friction in the system and submergence of thediffusers (e.g., 10 p.s.i.g.). Power costs vary between about 0.25 and1.6 k.w.h. per lb. BOD removed, and average about 0.56 k.w.h. per lb.BOD removed.

It has long been recognized that the use of air as an oxygen sourceimposes a serious limitation on the rate of oxygen dissolution which canbe sustained. Air contains only 20.8% oxygen and its other constituentsare inert to the biochemical reactions. In practice, the dissolvedoxygen is consumed from the mixed liquor by the bacteria so rapidly thatthe DO levels economically achievable with air aeration are suppressedbelow safe levels for a healthy, profuse growth of desirable aerobicbacteria. Anaerobic and faculative strains of bacteria may develop whichcause odors and extend treatment time.

High solids levels in the aeration zone are also beneficial toBOD-containing water aeration because the rate of BOD removal becomeshigher and the rate of excess sludge production becomes lower. However,high solids levels result in more rapid uptake of DO by the biomass, andin deference to the limited rate of dissolution of oxygen from air,waste treatment practitioners have deliberately reduced and suppressedactive solids levels in the mixed liquor. When the solids level isreduced, the rate of BOD removal is decreased and treatment tanks remainlarge in order to retain the waste for the requisite time period (3-6hours) necessary for purification.

The rate of oxygen dissolution can be increased by more violentagitation of the body of mixed liquor using surface aerators, beatersand submerged turbines. However, severe agitation breaks up anddisperses the flocculant agglomerates so that after treatment, thesolids do not separate properly from the effluent. Moreover, the solids,when gravity-settled, possess a high specific volume (Mohlman SVI) andthe necessary recycle of such solids as inoculant represents a severehydraulic burden on the system. Under conditions of low DO and lowsolids levels, the floc particles are small and fragile and areparticularly susceptible to dispersion by attrition. Attempts to"overpower" the aeration system have also led to prohibitive investmentand operating costs of the equipment involved.

It can be seen that the use of air as an oxygen source for biochemicalreactions imposes serious penalties on the method. It has been proposedto use pure oxygen or oxygen-enriched air for aeration as a means ofincreasing dissolution rates. With pure oxygen, it is possible toincrease the oxygen partial pressure difference between gas and liquidby five-fold. Many attempts have been made to utilize oxygen-enrichedaeration gas but without commercial success. In some of these attempts,the oxygen-enriched gas was merely substituted for air using equipmentand procedures common to air aeration. The high cost and low economy ofthese efforts resulted from the ineffective utilization of the oxygenwhich unlike air, is not "free" from the atmosphere. For example, whenpure oxygen is sparged or diffused in the normal way into a conventionaltreatment tank for municipal sewage, only about 5-10% of the oxygen isconsumed (i.e. Dissolved and utilized) and the remainder escapes to theatmosphere.

One of the best known of the prior art attempts to employoxygen-enriched aeration gas is the bioprecipitation process wherein afraction of the effluent from a combined reactor-clarifier is mixed withthe influent, oxygenated to near saturation, and then returned to thebase of the reactor. The reactor contains a blanket of active solids andthe highly oxygenated liquid rises slowly through the blanket, therebytransferring its organic pollutants to the bacterial floc and alsosupplying the needed oxygen for assimilation. The influent plus recycleeffluent is oxygenated by downflow through a countercurrent gas-liquidcontacting column--the oxygen-rich aeration gas being introduced at thebase of the column and vented at the top. Although the countercurrentcontactor is probably the most efficient mass transfer device for mostchemical processes, it too has failed to achieve the economy necessaryfor oxygen aeration of a biochemical oxidation process. Only 20 to 25%consumption of the feed oxygen has been realized.

One reason for the low utilization of the bioprecipitation process isthe very fact that the DO level in the oxygenator or contactor waspushed to near saturation with the result that the oxygen partialpressure driving force essentially vanished in the lower levels of thecolume. Simultaneously, the CO₂ and N₂ impurities stripped from theliquid severely depressed the oxygen partial pressure driving force inupper levels of the column. These combined factors prevented dissolutionof an economical, high fraction of the oxygen introduced. The nearapproach to DO saturation is a necessary objective of the process, sincethe full DO supply to treat the BOD-containing water must be containedand "carried" in the flow of diluted influent to the reactor.

It is an object of this invention to provide an improved method fortreating BOD-containing water by oxygen-enriched aeration gas.

Another object is to provide a method characterized by relatively highconsumption of oxygen in the aeration gas.

Still another object is to provide a method characterized by relativelyhigh oxygen consumption, high dissolved oxygen in the mixed liquor, andhigh solids concentration.

Other objects and advantages of this invention will be apparent from theensuing disclosure and appended claims.

SUMMARY

This invention relates to a method for treating BOD-containing water byoxygen-enriched aeration gas in contact with active biomass.

In this method, BOD-containing water, biomass and feed gas comprising atleast 60% oxygen (by volume) are mixed in an aeration zone to formliquor. The mixing is continued while simultaneously maintaining: (a)the oxygen feed gas to mixing plus gas-liquor contact energy ratio at0.03-0.40 lb. moles oxygen per horsepower hour of energy supplied, (b)the aeration gas above said liquor at oxygen partial pressure of atleast 300 mm. Hg but below 80% oxygen (by volume) while consuming atleast 50% (by volume) of the feed gas oxygen in the liquor, (c) thedissolved oxygen concentration of the liquor at below 70% of saturationwith respect to the oxygen in the aeration gas but above about 2 p.p.m.and (d) continuously recirculating one of the aeration gas and liquorfluids in intimate contact with the other of said fluids in saidaeration zone. Oxygenated liquor is thereafter withdrawn from theaeration zone.

This method may be used to treat municipal waste in a mannersignificantly more efficient .[.(in terms of oxygen consumption).]..Iadd.than the widely used air aeration treatment or than could beachieved in previously proposed oxygen aeration treatment methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between both oxygenconsumption and aeration gas oxygen concentration as ordinates versusoxygen feed gas rate/energy supplied ratio as the abscissa, for amunicipal-type waste water having 250 p.p.m. BOD, dissolved oxygenconcentrations of 2 p.p.m. and 8 p.p.m. and 99.5% oxygen feed gas.

FIG. 2 is a graph similar to FIG. 1 but for 80.0% oxygen feed gas.

FIG. 3 is a graph similar to FIGS. 1 and 2 but for 60.0% oxygen feedgas.

FIG. 4 is a graph similar to FIG. 1 but for an industrial-type wastewater having 2500 p.p.m. BOD using 99.5% oxygen feed gas.

FIG. 5 is a graph similar to FIG. 4 but for 80.0% oxygen feed gas.

FIG. 6 is a graph similar to FIGS. 4 and 5 but for 60.0% oxygen feedgas.

FIG. 7 is a graph showing the total annual aeration cost as the ordinateversus oxygen feed gas rate/energy supplied ratio as the abscissa, fordissolved oxygen concentrations of 2 p.p.m. and 8 p.p.m with themunicipal-type waste water and 99.5% oxygen feed gas of FIG. 1.

FIG. 8 is a graph similar to FIG. 7 but for 60% oxygen feed gas.

FIG. 9 is a shematic view taken in cross-sectional elevation ofapparatus including a single submerged agitator and sparger assemblywithin an aeration chamber, and a clarifier arranged to practice oneembodiment of the method of this invention.

FIG. 10 is a schematic view taken in cross-sectional elevation ofapparatus characterized by a multiplicity of submerged agitators andspargers all positioned within the same aeration chamber to practiceanother embodiment, and

FIG. 11 is a schematic view taken in cross-sectional elevation ofapparatus to practice still another embodiment characterized by amultiplicity of aeration chamber each having a surface-type mixer andoxygen feed gas introduction means and arranged for staged flow ofoxygenated liquor.

DESCRIPTION OF PREFERRED EMBODIMENTS

Practice of the improved oxygen aeration method requires the use of afeed gas of at least 60% oxygen purity. As will be explained laterherein, the oxygen content of the gas in the aeration zone issignificantly lower than the feed gas owing to the accumulation of inertgases in the zone. Therefore, a substantial oxygen concentration marginshould be provided in the feed gas to offset the mixing losses whichoccur in aeration and to maintain a high oxygen partial pressure incontact with the liquid.

The aeration gas must be held in the aeration or contact zone within thebiochemical reactor, isolated from the atmosphere for a time sufficientfor the dissolution of a large fraction of its contained oxygen into theliquor. During the retention period of the gas, at least one of thefluids (gas or liquor) is recirculated within the zone and contactedagainst the other. Devices such as packed columns which afford onlymomentary, once-through contact between the gas and liquid do notprovide enough retention time. Even if "dosed" to saturation, theBOD-containing liquid to be treated would not hold all the oxygen insolution required for the reaction, and as will be describedhereinafter, the oxygen gas to liquor dissolution process is not to bedriven to near-saturation nor should the dissolved oxygen aftertreatment be wholly depleted from the oxygenated liquor. Hence, the timethat the fluids are in contact is extended so that the rate ofdissolution keeps pace with, but does not greatly exceed the rate of DOconsumption.

In the aeration zone, a large interfacial area is generated between gasand liquor to promote rapid dissolution. However, this area must beproduced in a manner which avoids a close approach to oxygen saturationin the liquid bounding the interfacial area. This is accomplished byproducing the interfacial area in a large volume of liquor so that notmore than a thin film of liquor at the interface will be nearsaturation, and so that the DO gradient from the interface to the bulkliquor will be high. Preferably, the liquor phase in the aeration zoneshould be continuous, or should approach continuity. Small bubbles ofgas in the liquor constitute a desirable interfacial system, and surfaceaeration conducted by throwing relatively massive spouts or sheets ofliquid into the gas is satisfactory. A liquid spray should be avoidedbecause a droplet possesses a large surface area and a small volume ofliquid.

As stated previously, the interfacial area should be produced in theliquor contained within the biochemical reactor. All the liquor in thereactor should participate in gas-liquor contact so that the DO can bereplenished as it is consumed throughout the mixed liquor.

In order to preserve sufficient driving force for high-rate dissolution,the average mixed liquor DO level should not be forced above 70% ofsaturation with respect to the aeration gas oxygen purity, temperatureand pressure prevailing in the aeration zone. Preferably, the DO levelshould be less than 35% of saturation. Avoiding excessive DO level inthe bulk liquor assures a high total DO difference between thegas-liquor interface and the bulk liquid so that the DO is dispersedrapidly from the interface. Such dispersion proceeds by diffusion and bymixing, and both mechanisms become more rapid as the DO gradientincreases at the interface.

The oxygen concentration of the aeration gas is significantly lower thanthe feed gas owing to the accumulation of gases such as nitrogen, CO₂and argon. In the method of this invention, the accumulation of inertsis limited by venting gas from the aeration zone, continuously orintermittently, so as to maintain an oxygen partial pressure in theaeration zone as previously indicated of at least 300 mm. Hg andpreferably at least 380 mm. Hg. Such concentration is needed, not onlyto maintain a high rate of dissolution but also to insure the growth ofheavy, settable agglomerates of biomass.

If the oxygen feed point and the "spent" gas vent point are remote fromone another, and if the flow channel between the two points is somewhatrestricted, then the oxygen content of the gas will change significantlyalong the flow channel. The oxygen content will be highest at the feedpoint and lowest at the vent point. Where this situation exists, theforegoing minimum oxygen partial pressures refer to the zone of lowestoxygen content, or the venting region.

One important factor in control of the oxygen partial pressure in theaeration zone is the rate at which oxygen feed gas is introduced. Otherfactors remaining equal, an increase in the rate of oxygen introductionwill increase the oxygen partial pressure in aeration and vice versa.Certain factors will tend to oppose this trend: as the partial pressurerises, the biochemical reaction rate will often increase and gaseousby-products will evolve more rapidly. Thus, a greater total quantity ofinerts must be vented per unit time along with its complement of oxygen.More importantly it has now been recognized that with higher oxygen feedrate, each unit volume of vent gas will contain more oxygen due to thehigher oxygen partial pressure in the aeration zone. Accordingly, thepercentage of the oxygen which is consumed in the liquor will tend todecrease. Stated in another manner, during the venting of a givenquantity of inerts the amount of oxygen wasted will increase as theoxygen content of the aeration gas increases.

This invention utilizes the foregoing relationship by limiting theaeration gas to less than about 80% oxygen and preferably less thanabout 65% oxygen (by volume). As in the instance of the lower limit ofoxygen partial pressure, the upper composition limit refers to theregion of the aeration zone from which the inerts are vented. If thefeed-to-vent gas flow channel sustains an oxygen purity gradient, asdescribed previously, then upstream regions in the aeration zone willadvantageously contain aeration gas of higher oxygen content than thevent gas.

It will now be evident from the foregoing explanation that the desirefor high oxygen partial pressure in aeration and for high percentageutilization of the feed gas oxygen are conflicting objectives. Thistreatment method resolves the apparent dilemma by adherence to theforegoing lower limit of oxygen partial pressure and upper limit ofoxygen content in the aeration gas. The method can be controlled tomaintain operation between these limits and the rate of oxygenintroduction is one factor in achieving such control.

In this biochemical treatment method, at least 60% oxygen feed gas issupplied to the aeration zone, energy is supplied for contacting the gasand liquor, and a portion of the aeration gas in the form of a gaseousresidue of undissolved oxygen and accumulated impurities is vented. Therelationship between oxygen feed gas flow rate and energy input is afurther important factor of the invention. If more contacting energy issupplied with each volume of oxygen feed gas, then a larger fraction ofits oxygen will be dissolved and a lower fraction will be wasted in thevent gas. Energy cannot be increased without limit because eachadditional percentage point of oxygen utilized must be dissolved fromgas of progressively lower oxygen partial pressure. Moreover, with agiven quantity of liquor in the aeration zone to receive the dissolvedoxygen, an increase in energy will drive the DO level closer tosaturation and the rate of dissolution will drop. Hence, incrementalenergy affords diminishing returns. As a further restraint, it waspreviously indicated that excessive mixing energy will damage theflocculant biomass and impair subsequent separation. It has now beendiscovered that the amount of oxygen supplied bears a close relationshipto the amount of energy supplied. High oxygen partial pressure andmixing energy both provide driving force for dissolution, but by thisinvention the forces are employed in balance to achieve remarkablyhigher percent utilization than attainable by the prior art with highoxygen partial pressure in the aerator zone. This balance is achieved bymaintaining the oxygen feed gas to mixing plus gas-liquor contact energyratio as 0.03-0.40 lb. moles oxygen per horsepower-hour of energysupplied. In a preferred embodiment the ratio is 0.1-0.2 mole oxygen perhorsepower-hour of energy supplied.

The energy supplied to the aeration zone must of course be usedefficiently to generate the gas-liquor interfacial area required foroxygen solution, i.e., the gas-liquor contact energy. Mixing energy mustalso be employed to hold the solids uniformly in suspension and tocirculate the mixed liquor repeatedly through the gas-liquor contactor.Many types of aeration devices are commercially available, andgenerally, they are rated according to a "standard air transferefficiency." The latter rating specifies the lbs. oxygen which thedevice will dissolve from air into zero-DO tap water per horsepower-hourat 20° C. and 1 atmosphere pressure. An aeration device should be chosenwhose air transfer efficiency is at least 1.5 and preferably 2.4 lb.oxygen/horsepower hour in order that the oxygen may be dissolved rapidlydespite the relatively small volume of gas fed to the system, and inorder that the heavy floc will not be damaged and dispersed. In view ofthe small volume of oxygen-enriched gas involved (relative to the volumeof air commonly used), it is preferable to employ a combination of amechanical agitator for liquor stirring (mixing energy) and a submergedgas diffuser for gas-liquor contacting. However, some surface typeaerators of the splash type will perform both functions in asatisfactory manner.

Within the above stated range of 0.03 to 0.40 lb. mole oxygen/horsepowerhour, the oxygen and power supplied should be matched for best economydepending upon the applicable costs of aeration equipment, power andoxygen. Notwithstanding variations in these cost elements, aeration inaccordance with this invention can achieve at least 50% utilization ofthe oxygen supplied while maintaining an oxygen partial pressure severaltimes greater than obtained with air aeration and while consumingsubstantially less power than required for dissolution of an equalquantity of oxygen from air.

FIG. 1, 2 and 3 show computed results based on information confirmed byoperating data in a pilot plant used for testing municipal waste withoxygen-enriched aeration gas. The computed results are for a singlestage aeration system handling mixed liquor with a 2-hour solidsresidence time, with 4000 p.p.m. volatile suspended solids content(MLVSS), and with a waste feed BOD strength of 250 p.p.m. The biomassfor the aeration zone is provided by recycling activated sludge from aclarifier which in turn receives oxygenated liquor from the aerationzone. Six sets of data are included for oxygen feed purities of 60%, 80%and 99.5% and for DO levels of 2 p.p.m. and 8 p.p.m. In each data set,the power requirement and percent oxygen consumption were determined forvarious oxygen feed gas rates. FIG. 1 summarizes the results with 99.5%oxygen feed, and FIGS. 2 and 3 with 80% and 60% oxygen feed,respectively.

Referring to FIG. 1 for 99.5% oxygen feed, it is seen that high percentoxygen consumption can be obtained while maintaining high oxygen partialpressure in the aeration zone. The oxygen partial pressure curves,plotted on the right-hand ordinate, provide an indication of the lowerlimit of oxygen feed rate which will still produce a lower limit partialpressure of at least 300 mm. Hg. oxygen in aeration. The percentconsumption curves, plotted on the left-hand ordinate provide anindication of the upper limit of oxygen feed rate which will stillpermit at least 50% utilization of the oxygen in the feed. Extrapolationof the partial pressure curves to the minimum pO₂ of 300 mm. Hgindicates that feed rates as low as about 0.075 lb. moles O₂ /HP-hr. arepermissable for 8 p.p.m. DO, and as low as about 0.095 lb. moles for 2p.p.m. DO. Similarly, the percent consumption curves can be extended toabout 0.21 lb. moles O₂ /HP-hr. before falling below the minimum 50%consumption. The percent oxygen consumptions corresponding to 300 mm. Hgpartial pressure are above 90% for both DO levels. FIG. 1 also showsthat for the aeration gas oxygen concentration upper limit of 80% (about600 mm. Hg at 1 atmosphere) the oxygen utilization drops to about 60%for both DO levels.

FIG. 2 for 80% oxygen feed shows minimum feed rates corresponding to 300mm. Hg partial pressure of about 0.07 and 0.10 lb. moles/HP-hr. for 8and 2 p.p.m. DO, respectively. Maximum feed rates corresponding to 50%consumption are about 0.18 and 0.20 lb. moles O₂ /HP-hr. for 8 and 2p.p.m. DO, respectively. Thus, the range of feed rates which permitoperation within the limits of the invention is still ample. However, itwill be noted that about 80% consumption is the best that can beachieved with partial pressure above 300 mm. Hg. It is also significantthat if the oxygen partial pressure was increased by higher O₂ feed rateto achieve 80% oxygen (600 mm. Hg), the corresponding percentconsumption would be far below the required 50% and thereby outside thescope of this method.

A study of FIG. 3 for 60% oxygen feed will show that operation accordingto the invention requires precise control of the oxygen feed rate with avery narrow range. The partial pressure curve for 8 p.p.m. DO risesabove 300 mm. at about 0.12 lb. moles O₂ /HP-hr. and the percent oxygenconsumption curve falls below 50% at about 0.10 lb. moles O₂ /HP-hr. For2 p.p.m. DO the same near-coincidence of minimum and maximum feed ratesoccurs at about 0.15 lb. moles O₂ /HP-hr. Thus, dropping the feed purityto 60% has greatly narrowed the operating range. The best (and only)percent consumption which can be achieved with a partial pressure above300 mm. Hg is 50%. Upon further reduction in oxygen feed purity, the"minimum" feed rate established by 300 mm. partial pressure would behigher than the "maximum" established by 50% consumption and therequirements of this method could not be met at any feed rate.

To illustrate use of the FIGS. 1-3 curves, Table A compares the threefeed gases at an oxygen gas feed rate of 0.11 lb. moles O₂ HP-hr. and aDO level of 8 p.p.m.:

                  TABLE A                                                         ______________________________________                                                            Aeration                                                               O.sub.2                                                                              zone O.sub.2                                                           con-   partial                                                                sumption,                                                                            pressure,                                                              percent                                                                              mm.                                                       ______________________________________                                        Feed gas:                                                                     60%, O.sub.2   ˜50                                                                                300                                                 80%, O.sub.2   70       350                                                   99.5%, O.sub.2 90       450                                                   ______________________________________                                    

Similarly, Table B compares feed rates and percent oxygen consumptionfor the three feed gas purities at a uniform partial pressure of 300 mm.Hg:

                  TABLE B                                                         ______________________________________                                                     DO=8 p.p.m.                                                                          O.sub.2 con-                                                           Feed   sumption,                                                              rate   percent                                                   ______________________________________                                        Feed gas:                                                                     60%, O.sub.2   .112     50                                                    80%, O.sub.2   .067     80                                                    99.5% O.sub.2  .072     95                                                    ______________________________________                                    

The FIG. 1 curves also illustrate the low sensitivity of the method tovariations in DO-level when an 99.5% oxygen feed is employed. The twosets of curves for 2 and 8 p.p.m. DO are very close together so thatincreasing the DO-level in this range would have no significant effecton either percent oxygen consumption or aeration zone oxygen partialpressure. However, the FIGS. 2 and 3 curves for 80% and 60% oxygen feedboth show appreciable sensitivity to DO-level changes. Because DO levelsdo in fact vary in waste treatment plants due to changes in feed flowsthe BOD level, this insensitivity in oxygen consumption represents animportant advantage of the preferred embodiment over that wherein thefeed gas comprises at least 90% oxygen. That is, a 90% oxygen feed gaswould be relatively insensitive to changes in DO levels as compared to60% oxygen.

The air standard transfer efficiency of the aerator upon which FIGS. 1-3are based in between 3 and 3.5 lb. O₂ /HP-hr. The effect of using a lessefficient device would depress the percent consumption curves and raisethe partial pressure curves, whereas a more efficient device would havethe opposite effect. However, the conclusion drawn with respect to thelimited utilization at 60% oxygen feed would not be materially affected.

It should also be understood that the power consumption to which theoxygen feed rate is related in the abscissas of the figures is the totalpower required for solid-liquid mixing and for gas-liquor contact. Forconventional air diffusers, the total power is usually consumed by aircompressors. In other aerator systems, a part of the power is consumedby mechanical agitators to hold the solids in suspension, and theremainder of the power is used by compressors which supply gas tosubmerged spargers. In still other surface aerators, all the power isconsumed in mechanical agitation of the liquor.

Because the mixing power is included in the ratio of the oxygenfeed/total power, the latter values are influenced by the treatment orrelention time of the mixed liquor solids in aeration. Mixing energymust be supplied at steady rate, and if treatment time is long, thebasins will be large and mixing energy consumption relatively high.Also, if treatment time is long, the rate of oxygen feed may berelatively low, reflecting slow volumetric uptake rate of DO. Thus, withextended treatment time, mixing energy constitutes a larger fraction ofthe total energy, and the ratio of oxygen feed/power is relatively low.For short treatment time, the opposite is true.

FIGS. 4, 5 and 6 are similar to FIGS. 1, 2 and 3, respectively, exceptthat the mixed liquor MLVSS is 6000 p.p.m. and the waste feed BODstrength is 2500 p.p.m. While the FIGS. 1-3 typify low strengthmunicipal wastes, the FIGS. 4-6 typify treatment of higher strengthindustrial wastes by the instant method. One effect of increasing theBOD strength (and the MLVSS) is to increase the uptake rate of dissolvedoxygen. At a given oxygen feed rate per horsepower-hour, this increasesthe percent oxygen consumption and decreases the oxygen partialpressure.

With reference to FIG. 4 for 99.5% oxygen feed gas and 8 p.p.m. DO, theoxygen feed rate to maintain 300 mm. Hg oxygen partial pressure may beapproximated from the curve and is about 0.10 lb. mole O₂ /HP-hr. andthe oxygen feed rate for 50% consumption is 0.42 lb. mole O₂ /HP-hr. Itwill be apparent that for this particular embodiment the aeration gasmust be maintained considerably below 80% oxygen (600 mm. Hg at 1atmosphere) to avoid exceeding the 0.40 upper limit on the ratio ofoxygen feed rate/energy supplied and dropping below the 50% oxygenconsumption lower limit. For 2 p.p.m. DO, the minimum feed rate is about0.14 lb. mole O₂ /HP-hr. for the aeration gas minimum of 300 mm. Hgoxygen partial pressure and the maximum is 0.52 lb. mole O₂ /HP-hr. for50% consumption, but the latter is above the upper limit of thisinvention for oxygen feed gas rate/energy supplied. The permissibleoperating range is large and includes high percent consumption andcorresponding high oxygen partial pressures.

FIG. 5 for 80% oxygen feed gas shows minimum and maximum oxygen feedrates of 0.12 and 0.28 lb. mole O₂ /HP-hr., respectively, for 8 p.p.m.DO, and of 0.18 and 0.36 lb. mole O₂ /HP-hr., respectively for 2 p.p.m.DO. Again 80% oxygen in the aeration gas is precluded by low oxygenconsumption and high oxygen feed gas rate/energy supplied.

With reference to FIG. 6 for 60% oxygen feed and 2500 p.p.m. BOD wasteliquid, it is clear that the oxygen feed range for practicing theinvention has vanished at both 2 and 8 p.p.m. DO-levels in the liquor.For example, at 8 p.p.m. DO-level, the oxygen feed rate must be at least0.24 lb. mole O₂ /HP-hr. to reach a partial pressure of 300 mm. Hg, yetthe oxygen feed rate cannot be above 0.15 lb. mole O₂ /HP-hr. and stillobtain an oxygen consumption of 50% or higher. Returning to FIG. 3, for60% oxygen feed and 250 p.p.m. BOD waste liquid, the same "negative"operating range is seen to exist, although there is much less disparitybetween the limits imposed by oxygen consumption and oxygen partialpressure. While the disparity for high strength, 2500 p.p.m. BOD liquidis 0.15-0.24= -0.09 lb. mole O₂ /HP-hr., the disparity for 250 p.p.m.BOD liquid is only 0.10-0.12=-0.02 lb. mole O₂ /HP-hr. It is apparentthat a still further reduction in the BOD strength of the waste liquidfed to the process, as may readily exist in dilute municipal wastes,will rsult in a finite operating range applicable to 60% oxygen feed.However, a comparison of the 80% and 60% oxygen feed curves for eitherwaste strength (FIGS. 2 and 3 or FIGS. 5 and 6) shows that a furtherreduction in oxygen feed purity below 60% is not feasible since therequirements for at least 50% oxygen consumption and at least 300 mm. Hgoxygen partial pressure cannot be met simultaneously.

FIGS. 7 and 8 illustrate the variation in cost of treating 250 BOD wasteat different oxygen feed rates. The "cost units" shown on the right-handordinate are relative values only, but they reflect the total costs ofaeration, including investment in aeration equipment, depreciation,maintenance and power. FIG. 7 for 99.5% oxygen feed shows very sharpoptimums at about 0.12 lb. mole O₂ /HP-hr., where the costs formaintaining 2 and 8 p.p.m. DO-levels are about 27 and 28 cost units,respectively. The corresponding values of oxygen consumption are about90%. FIG. 8 for 60% oxygen feed shows optimums for 2 and 8 p.p.m. DO atabout 0.11 lb. mole O₂ /HP-hr. where the aeration costs are about 50 and60 units, respectively. Thus, the location of the optimums of FIG. 8establish 50% oxygen consumption as the minimum consistent with lowestcost treatment of BOD-containing liquids.

By comparing FIGS. 7 and 8, it is also apparent that the method using99.5% oxygen feed gas is relatively insensitive to change in DO-level.In contrast, the use of 60% oxygen feed gas entails significantly higheroperating costs with increasing DO-level.

Referring to FIG. 9, BOD-containing water, as for example municipalsewage, enters chamber 10 through conduit 11. A source (not shown) ofoxygen comprising at least 60% oxygen is provided and the oxygen gas isflowed therefrom through conduit 12 having control valve 13 therein tochamber 10. The latter is provided with gas-tight cover 14 to maintainan oxygen-enriched aeration gas environment over the liquor. Recyclingsludge is also introduced to chamber 10 through conduit 15, although theBOD-containing feed water and sludge may be mixed prior to introductionin the chamber if desired.

The aforementioned streams are intimately mixed to form liquorpreferably having volatile suspended solids content (MLVSS) of at least3000 p.p.m. in chamber 10 as the aeration zone. This mixing is bymechanical agitation means 16 driven by motor 17 having a shaft passingthrough seal 18 in the cover 14. Although the agitation means maycomprise one or more impellers located near the liquor surface, it isillustrated as positioned below the surface. In this particularembodiment, oxygenating aeration gas disengaged from the liquor bodyinto the overhead gas space is withdrawn through conduit 19 by blower 20for compression and return through conduit 21 to submerged sparger ofdiffuser 22 preferably positioned beneath agitator 16. That is, theaeration gas is continuously recirculated in intimate contact with theliquor body in chamber 10. Blower 20 is driven by a motor (notillustrated) representing the gas-liquor contact energy, and ispreferably provided with controls to permit adjustment of its speed ofrotation. Oxygen-depleted or spent oxygenation gas is discharged fromchamber 10 through restricted flow conduit 23 which may also be providedwith flow control valve 24.

To practice the method of this invention, the BOD-containing water,oxygen-rich feed gas and sludge are mixed to form the mixed liquor, andthe oxygenating gas is continuously recirculated into the liquor fordissolution. Inert gases such as nitrogen entering with theBOD-containing water and with the oxygen-rich feed gas, and gases suchas CO₂ produced in the biochemical reaction are evolved and collectedwith unconsumed oxygen in the space above the liquor. This aeration gashas an oxygen partial pressure of at least 300 mm. Hg and preferably atleast 380 mm. Hg. The oxygen-rich gas may be continuously introduced tochamber 10 through conduit 12 during the mixing step, or the gas flowmay be terminated when mixing is started. The oxygen-depleted aerationgas may be continuously or intermittently discharged from the overheadspace through conduit 23.

The liquor level in enclosure 10 is controlled by weir 25 whichdischarges into overflow trough 25 and thence through discharge conduit27. The dissolved oxygen level in the oxygenated liquor formed in themixing step is maintained at below 70% of saturation with the oxygen inthe aeration gas and is preferably at least 2 p.p.m. Adjustments in DOlevel may be accomplished by varying the rate of oxygen-rich feed gasflow using valve 13 in conduit 12 thereby increasing or decreasing theoxygen partial pressure in the enclosure 10 gas space. The DO level mayalso be adjusted by varying the power input and speed of rotation ofblower 20, thereby increasing or decreasing the rate of diffusion ofoxygenated gas into the liquor. The DO level may also be controlled byvarying the retation time of the liquor in chamber 10. All otherparameters being constant, a longer liquor retention time tends toprovide a higher DO level.

At the end of the mixing step for example 20 to 180 minutes duration,oxygenated liquor is discharged through conduit 27 to within a centralconcentric baffle 28 of clarifier 29. Baffle 28 preferably extends fromabove the liquid level to a point intermediate this level and theclarifier's conical bottom. Motor 30 drives a slowly rotating rake 31across the clarifier bottom to prevent "coning" of the dense settledsludge. The purified supernatant liquid overflows weir 32 into trough 33and is discharged through conduit 34. The sludge is withdrawn from theclarifier bottom through conduit 35 and at least a portion thereof ispressurized by pump 36 for recycling in conduit 15 to enclosure 10 forinoculation of the incoming BOD-containing water. Any sludge not neededfor recirculation is discharged through bottom conduit 37 having controlvalve 38 therein.

FIG. 10 illustrates different apparatus for practicing this method,employing a multiplicity of submerged agitators 16a-e and recirculationoxygen enriched gas spargers 22a-e spaced longitudinally from end-to-endof oxygenation enclosure 10. After premixing, BOD-containing water andrecycling sludge are introduced through conduit 11 at one end ofenclosure 10. The resulting liquor is mixed with oxygen-rich gasintroduced through conduit 11 and the oxygenated liquor discharged fromthe opposite end of enclosure 10 through conduit 27 to a clarifier (notshown). Oxygen-depleted gas is also discharged from the space above theliquor level and at this opposite end through conduit 23 oxygentatingaeration gas is withdrawn through longitudinally spaced conduits 19a-efor pressure recirculation through blowers 20a-e and spargers 22a-e in amanner analogous to the FIG. 9 embodiment.

Enclosure 10 may be designed so that its length is very large relativeto its width and depth. For a given enclosure volume such geometryincreases the velocity of liquor flow from feed end to discharge end,and suppresses backmixing of liquor from downstream zones into upstreamzones. Such suppressed backmixing or plug flow is beneficial whenmultiple mixing means are employed in the instant method. Whenback-mixing is prevented, the food/biomass ratio (lbs. BOD₅ /day×lb.MLVSS) is high at the feed end of the enclosure where the BOD-containingwater enters, and is low at the discharge end where the oxygenatedliquor overflows to the clarifier. Both of the local conditions arebeneficial to complete and high rate bio-oxidation.

It will be apparent from the foregoing description of FIG. 10 that theliquor is oxygenated in a series of stages from the feed to thedischarge end of container 10 even though the stages are not physicallypartitioned from each other. If container 10 is designed with smalllateral cross-sectional area in the gas space beneath cover 14, asimilar staged or plug flow effect can be realized in the oxygenationgas flow from feed to discharge end. This also promotes virtuallycomplete BOD removal at high flow rate, because a substantially higherpartial pressure of oxygen can be maintained over the liquor at the feedgas end. Another advantage of staged gas flow is that the inert gaseousimpurities can be discharged from the opposite end in a smaller volumeof aeration vent gas. As the oxygenation gas flows from end-to-end ofthe enclosure 10 the rate of oxygen dissolution into the liquor issubstantially greater than the rate of inert gas evolution from theliquor. Accordingly, the volume of the oxygenating gas streamprogressively diminishes and its fractional content of inerts increasesfrom the gas feed to discharge end. It is desirable to aerate the highfood/biomass zone (where BOD-containing water is introduced) with thegas of highest oxygen content available because the oxygen demand isgreatest in this zone. Conversely, the oxygen demand is lowest at theoxygenated liquor discharge region and it is preferable to employ theavailable aeration gas of lowest oxygen content in this region.Accordingly, in embodiments of this invention wherein liquor is flowedthrough a multiplicity of zones for staged mixing with oxygen-enrichedaeration gas, it is also preferred to flow the aeration gas concurrentlywith the liquor from stage-to-stage with the gas of highest oxygencontent mixing with the water of highest BOD.

The FIG. 11 apparatus illustrates a mixing chamber 10 divided into fourseparate compartments or stages 30a, 30b, 30c and 30d. Partition 31a-bextends from bottom to top of chamber 10 to separate first and secondcompartments 30a and 30b. Similarly, partition 31b-c separates secondand third compartments 30b and 30c, and partition 31c-d separates thirdand fourth compartments 30c and 30d. Restricted opening 32a-b providesflow of partially oxygenated liquor from first compartment 30a to secondcompartment 30b, restricted opening 32b-c provides flow of furtheroxygenated liquor from second compartment 30b to third compartment 30c,and restricted opening 32c-d provides flow of still further oxygenatedliquor from third compartment 30c to fourth compartment 30d.

Oxygen-rich gas is introduced through manifold 12 and control valves13a, 13b, 13c and 13d in branch conduits to each of the fourcompartments for simultaneous mixing therein with BOD-containing liquor.These valves may for example, be responsive to a suitable variable suchas DO level in the liquor or gas composition with the compartment.Surface-type aerators 22a, 22b, 22c and 22d throw massive spouts orsheets of liquor into the aeration gas. Accordingly, these aeratorsprovide both liquid-solid mixing energy and the gas-liquor contactenergy for the aeration step. In contrast to this, the FIGS. 9 and 10subsurface-type units continuously recirculate the liquor (instead ofthe aeration gas) in intimate contact with the aeration gas in eachcompartment. Because the chamber walls and partitions confine the fluidswithin each compartment, surface mixers may be used in this embodimentwithout backmixing of the liquor thrown upward and outward of theimpeller. The oxygen-depleted aeration gas disengaged from the liquor isdischarged from each compartment through restricted flow conduits 23a,23b, 23c, and 23d. These conduits may be provided with flow controlvalves if desired.

An advantage of the FIG. 11 apparatus is the close approach to true plugflow of liquor. The liquor velocity through restricted openings 32a-b,32b-c and 32c-d is sufficient to prevent backmixing. The liquor in eachcompartment or stage is substantially uniform in composition and the BODcontent progressively declines from the liquor feed stage 30a to theliquor discharge stage 30d.

Although the biomass mixed with BOD-containing water in the apparatus ofFIGS. 9-11 is provided by recycling activated sludge, this is notessential to the practice of the invention. The aeration zone may in theform of a covered chamber positioned within and in open communication atits lower end with a body of BOD-containing water, e.g., a lagoon or afermentation tank. In this event, the biomass may be circulated bynatural flow and the aeration devices. In the lagoon waste treatmentembodiment part of the sludge (biomass) settles to the lagoon floor andmay be periodically withdrawn therefrom and removed by dredging means.

In a preferred embodiment of this method is which waste water is treatedby oxygenation in contact with sludge, waste water, sludge and feed gascomprising at least 90% oxygen (by volume) are mixed in an aeration zoneto form a liquor body having MLVSS of at least 3000 p.p.m. The mixing iscontinued while simultaneously maintaining (a) the oxygen feed gas tomixing plug gas-liquor contact energy ratio of 0.1-0.3 lb. mole oxygenper horsepower hour energy supplied, (b) the aeration gas above theliquor at oxygen partial pressure of at least 380 mm. Hg but below 65%oxygen while consuming at least 70% of the feed gas oxygen in theliquor, (c) the dissolved oxygen concentration of the liquor at below35% of saturation with respect to the oxygen in the aeration gas, and(d) continuously recirculating aeration gas in intimate contact with theliquor in the aeration zone. Oxygenated liquor is withdrawn from theaeration zone and separated into sludge and lean effluent. At least partof the sludge is recycled to the aeration zone.

Summarizing this invention, an aeration method is provided wherebyoxygen-enriched gas may be employed economically in biochemicaloxidation reactions. A high percentage of the oxygen fed to the reactionis utilized while maintaining a high partial pressure of oxygen in theaeration gas. Dissolution rates are high affording dissolved oxygenlevels, even with very high solids concentrations in aeration, wellabove those previously obtained economically. Power consumption andinvestment in aeration equipment are low and a heavy, fast-settling flocis produced which is not damaged during treatment.

Although certain embodiments have been described in detail, it will beappreciated that other embodiments are contemplated along withmodifications of the disclosed features, as being within the scope ofthe invention.

What is claimed is:
 1. In a method for treating BOD-containing water byoxygenation in contact with active biomass, the improvement comprising:mixing BOD-containing water, biomass and feed gas comprising at least60% oxygen (by volume) in an aeration zone .Iadd.of a reactor.Iaddend.to form .[.a.]. .Iadd.an oxygenated .Iaddend.liquor body and.[.continuing said mixing while.]. .Iadd.a body of aeration gas isolatedfrom the atmosphere above said liquor and continuously recirculating oneof the aeration gas and liquor fluids in intimate contact with the otherof said fluids in said aeration zone while venting a portion of theaeration gas to the atmosphere; and .Iaddend. simultaneouslymaintaining: (a) the oxygen feed gas to mixing plus gas-liquor contactenergy ratio of 0.03-0.4 lb. moles oxygen per horsepower hour of energysupplied, (b) the aeration gas above said liquor at oxygen partialpressure of at least 300 mm. Hg but below 80% oxygen while consuming atleast 50% of the feed gas oxygen in said liquor, (c) the dissolvedoxygen concentration of said liquor at below 70% of saturation withrespect to the oxygen in said aeration gas but above about 2 p.p.m. and.[.(d) continuously recirculating one of the aeration gas and liquorfluids in intimate contact with the other of said fluids in saidaeration zone; and thereafter.]. withdrawing oxygenated liquor from saidaeration zone.
 2. A method according to claim 1 in which said aerationgas is continuously withdrawn from said aeration zone and reintroducedto the body of said liquor.
 3. A method according to claim 1 in whichsaid feed gas comprises at least 90% oxygen.
 4. A method according toclaim 1 in which the dissolved oxygen concentration of said liquor ismaintained at below 35% of saturation.
 5. A method according to claim 1in which the oxygen partial pressure of said aeration gas is maintainedabove 380 mm. Hg.
 6. A method according to claim 1 in which said oxygenfeed gas to mixing plus gas-liquor contact energy ratio is maintained at0.1-0.3 lb. moles oxygen per horsepower hour of energy supplied.
 7. Amethod according to claim 1 in which the aeration gas is maintainedbelow 65% oxygen.
 8. A method according to claim 1 in which the volatilesuspended solids content (MLVSS) of the liquor is at least 3000 p.p.m.9. A method according to claim 1 in which municipal waste comprises saidBOD-containing feed water, said oxygenated liquor is separated intosludge and clean effluent, and at least part of said sludge is recycledto said aeration zone as said active biomass.
 10. In a method fortreating municipal waste water by oxygenation in contact with sludge,the improvement comprising.Iadd.: .Iaddend.mechanically mixing wastewater, sludge and feed gas comprising at least 90% oxygen (by volume) inan aeration zone .Iadd.of a reactor .Iaddend.to form .[.a.]. .Iadd.anoxygenated .Iaddend.liquor body having volatile suspended solids content(MLVSS) of at least 3000 p.p.m. .[.and continuing said mixingwhile.]..Iadd., and a body of aeration gas isolated from the atmosphereabove said liquor and continuously recirculating aeration gas inintimate contact with liquor in said zone while venting a portion of theaeration gas of the atmosphere; and .Iaddend.simultaneously maintaining:(a) the oxygen feed gas to mixing plus gas-liquor contact energy ratioat 0.1-0.2 lb. moles oxygen per horsepower hour of energy supplied, (b)the aeration gas above said liquor at oxygen partial pressure of atleast 380 mm. Hg but below 65% oxygen while consuming at least 70% ofthe feed gas oxygen in said liquor, (c) the dissolved oxygenconcentration of said liquor at below 35% of saturation with respect tothe oxygen in said aeration gas but above about 2 p.p.m..[., and (e)continuously recirculating aeration gas in intimate contact with saidliquor in said aeration zone.].; withdrawing oxygenated liquor from saidaeration zone and separating said oxygenated liquor into sludge andclean effluent; and recycling at least part of said sludge to saidaeration zone.